Removing Dependency from SDK Core Project

- Copied files required for Rest API SDK
- Removed PPApiContext and directly connected APIContext with PPConfigManager
- Removed duplicate data storage of configuration and credentials.
- Code Style Fixes
- Remove build.xml file as it is not required anymore
- Updated the samples
- Updated the documentations

tests/bootstrap.php

@@ -3,4 +3,6 @@
 // Include the composer autoloader
 $loader = require dirname(__DIR__) . '/vendor/autoload.php';
 $loader->add('PayPal\\Test', __DIR__);
-define("PP_CONFIG_PATH", __DIR__);
+if (!defined("PP_CONFIG_PATH")) {
+    define("PP_CONFIG_PATH", __DIR__);
+}